The invention relates to a method for measuring properties of a moving fiber web.
Further, the invention relates to an apparatus for measuring properties of a moving fiber web.
In the paper industry a traversing array of sensors is commonly used to measure properties of a moving fiber web during manufacture. The direction of traverse is normally substantially perpendicular to the direction of movement of the web. The sensors therefore measure properties of diagonal samples of the web, rather than the whole web. Typically, the speed of traverse is 20-40 centimeters per second, while the web moves at 10-30 meters per second. Measurements are made at substantially the same plurality of locations across the machine during each traverse, and may be made while traversing the web in one or both directions. The measurement from each such location is commonly termed a cell in the profile of measurements from a full traverse. Profiles measured in this way contain a combination of machine direction (MD) and cross-machine direction (CD) variations. Moreover, due to the scanning process, each CD location is infrequently sampled. Thus, scanning measurements provide only limited information about the MD and CD variability of the moving web. The method of measuring with scanning sensors requires that the web be unsupported in its passage through the scanning apparatus. Furthermore, since the traversing apparatus is stationary in the MD while the web may be moving at considerable speed, contact between the traversing sensors and the web is undesirable. As a result, the path of the web past the sensors may not be constant, and fluttering or other movement of the plane of the web relative to the sensors may compromise the accuracy of measurements, especially measurements of optical or surface properties of the web. This fluttering usually varies with web speed and other process operating conditions, and typically varies in degree across the web. Thus, the effect of web flutter on traversing measurements differs from time to time, and generally differs between the edges of the web and the centre of the web. A further disadvantage of this type of apparatus is the risk of web damage or breaks from contact between the moving fluttering web and the scanning sensors. In practice, such apparatuses can be utilized only in parts of the web manufacturing process where the web is sufficiently strong to support itself, and to survive occasional contact with the sensors due to fluttering. This prevents their use in, for example, the wet end of a paper machine.
The above mentioned technique employing traversing sensors is disclosed for example in U.S. Pat. No. 4,903,528 which further discloses methods which attempt to compensate for the diagonal path of the sensors relative to the web, and to estimate the web properties at locations in the web which were not measured. Such methods are of very limited efficacy, due to the incomplete representation of variation in web properties which is provided by traversing apparatus. In practice, the estimation is tolerably accurate only for wavelengths exceeding the MD web movement in a full CD traverse of the sensors. U.S. Pat. No. 4,319,847 discloses an apparatus which reduces the degree of sheet fluttering by employing directed jets to stabilize the web. This method reduces variation in the web path relative to the traversing sensors and thus can improve measurement accuracy for optical properties, but it does not completely eliminate web flutter. U.S. Pat. No. 5,047,652 discloses an alternative apparatus, comprising a backing roll which both stabilizes the web in a known constant curved path and provides optical standards for the traversing sensors.
Alternative techniques employ arrays of detectors deployed across the machine, measuring properties of the web at plural locations substantially simultaneously. Examples of such methods are disclosed in U.S. Pat. No. 4,565,444 including a variant method which employs multiple light pipes to a single detector. The technique disclosed in U.S. Pat. No. 4,565,444 overcome the drawbacks inherent in sequentially scanning across the web. However, they still scan the web in the MD, as the sensor apparatus remains stationary with respect to the moving web. The finite time constants of measurement devices cause the measurements to be an average of a significant length of the web in the MD, and this averaging time varies with the web speed. Typically, several milliseconds or dozens of milliseconds must be used for each measurement, during which time the web moves several dozens of centimeters. Thus, although the resolution of these measurement devices may be fixed and less than a centimeter in the CD, their resolution is dozens of centimeters and varying in the MD. Moreover, as the measurement device is stationary while the web is moving in the MD, contact between the apparatus and the web is undesirable. As a result, the above-mentioned problems of web flutter affect these measurement methods also. Similarly, as the web must also be unsupported in its passage through these apparatuses, they can be employed only where the web is sufficiently strong to support itself.
An object of the present invention is to provide a method and an apparatus which allow the above-mentioned drawbacks to be overcome.
The method of the invention is characterized by measuring the properties of the web with a sensor element which is arranged to a measuring roll which touches the web to be measured, wherein the circumferential speed of the measuring roll is arranged to substantially equal the web speed so that the web is substantially in non-slipping contact with the measuring roll.
Further, the apparatus of the invention is characterized by comprising at least one sensor element and a rotating measuring roll which touches the web to be measured, the sensor element being arranged to the measuring roll, wherein the circumferential speed of the measuring roll is arranged to substantially equal the web speed so that the web is substantially in non-slipping contact with the measuring roll.
The essential idea of the invention is that fiber web properties are measured with a measuring apparatus where at least one sensor element is arranged to a rotating roll which is in contact with the web to be measured, whereby the circumferential speed of the roll is arranged to substantially equal the web speed, the web being substantially in non-slipping contact with the roll. The idea of a preferred embodiment is that several sensor elements are arranged to the roll next to one another in the cross-machine direction. Furthermore, the idea of another preferred embodiment is that sensor elements are arranged to the roll one after another in the direction of its circumference, preferably in a regular grid pattern.
An advantage of the invention is that since the sensor element is continuously in contact with the same area of the web for the time sufficient for a reliable measurement, there is no MD averaging implicit in the measurement technique, except over the area of the sensor element. Further, when the spacing of sensor elements in the MD is fixed, the resolution of measurement in the MD is also fixed and does not vary with the web speed. The MD resolution is thus limited only by the dimensions of the sensor elements and high resolution may be easily achieved in MD measurements. When the measurements are made simultaneously at the plurality of CD locations, a true CD profile measurement is provided. Additionally, since the web is in contact with the roll, its position with respect to the sensors is known and constant, and does not vary either with CD position or with process operating conditions, so that web flutter does not affect the measurement technique. Further because the circumferential speed of the roll substantially equals the web speed the roll does not cause marks to the web or otherwise damage it. Yet another advantage is that the measuring roll can support the web during the measurement, so that the measuring apparatus can be situated in places where the web cannot support itself. For example, a measuring roll can be installed in the wet end of a paper machine.